Lignin is a kind of substance composed of polymerized aromatic alcohols, which exists in woody tissues. The main function of the lignin is to harden cell walls by forming interlaced nets, so as to form a main component of secondary walls. A benzene ring, an aldehyde group, a ketone group, a carboxyl group, a double bond etc. in a basic functional group contained in the lignin each have a certain ultraviolet absorption effect, so that the lignin can be applied to a wood-plastic composite to improve the ultraviolet aging resistance effect of the wood-plastic composite; moreover, the lignin contains a photosensitive group, for example a carbonyl group, is instable to light, and can achieve controllability of a degradation period of the wood-plastic composite through reasonable proportioning; in addition, the lignin is mainly positioned among cellulose fibers to achieve a compression resisting effect, so that the lignin can improve the performance of the wood-plastic material when it is added into the wood-plastic material.
Generally, photostabilizers, such as a light shielding agent, a radical scavenger, an ultraviolet absorbent, etc., UV-P (2-(2′-hydroxy-5′-methylphenyl)benzotriazole), UV-0 (2,4-dihydroxy benzophenone), UV-9 (2-hydroxy-4-methoxybenzophenone), UV-531 (2-hydroxy-4-n-octyloxybenzophenone), etc. are directly added into raw materials of the existing wood-plastic composite in order to improve the uvioresistant effect of the existing wood-plastic composite, and these photostabilizers are high in price. Compared with these photostabilizers, the lignin widely exists in nature, and a large amount of wastes are generated every year. Therefore, these wastes are combined with plant cellulose, the uvioresistant effect of the wood-plastic composite can be improved by using ultraviolet absorption groups therein, and the cost can be reduced while the environment is protected. In addition, the lignin contains a large amount of alcoholic hydroxyl groups and phenolic hydroxyl groups, and can improve the compatibility of the composite material, thereby improving the mechanical properties of the composite material.
At present, plant wood fiber, as a main component for producing wood-plastic composites, has been widely studied and applied in various kinds of dosages. An application number CN201310324655.X applies different types of plant fibers to production of wood-plastic materials. Application numbers CN204398445U and CN105315688A, in order to solve the problems of the application of agricultural straws, the shortage of wood flour, etc., focus on the research of lignin products based on agricultural straws and other plant fibers, and achieve a good effect. However, application of lignin as a second largest renewable resource to wood-plastic materials is rarely researched. In CN102051002A, enzymatic hydrolysis lignin is used to mix with wood fiber and polyolefin, so as to prepare a wood-plastic composite, but only the enzymatic hydrolysis lignin is used, and other lignins are not effectively utilized; in addition, all substances are mixed in a high-speed mixer, a lubricant, a coloring agent and the like therein are not good for modification grafting of a compatilizer on straw powder and lignin, so that the raw materials (the plastic, the straw powder, the lignin, the inorganic mineral powder, etc.) cannot be fully compatible and mixed together. Therefore, according to the present invention, experiments are carried out on the application of different lignins, and a two-stage in-situ modified granulation extrusion method is adopted, namely the raw materials are subjected to compatible blending in a first step, so that the raw materials are fully mixed, then other processing aids are added during extrusion granulation, and after granulation, the wood-plastic composite which has good mechanical properties, aging resistance and ultraviolet aging resistance can be prepared by extrusion.